Rinse fish fillets and pat dry.
Chop fillets and set aside.
Place potatoes in water in a saucepan over medium-high heat.
Cover, reduce heat and simmer until tender.
Remove from heat, drain and set aside.
In a skillet over medium-high heat, saute onions, mushrooms and bell pepper in margarine until onions are translucent.
Add flour and stir to blend well.
Gradually add milk, stirring constantly until smooth.
Add fish, black pepper, tamari and parsley.
Cook approximately 10 minutes, or until fish is tender.
Add cooked potatoes and sherry.
Heat and serve.